[OutOfHere]

There are at least three concerns from demineralized or acidic drinking water:

1. Drinking demineralized water results in an obvious multi-mineral deficiency, considering that we actually rely on drinking water for meeting a meaningful subset of our calcium, magnesium and other mineral intake. Even moderate supplementation won't correct it -- it takes a double dose to prevent it under RO.

2. Demineralized water causes minerals to be leeched from the body, also from the bones. This is to balance the water. It would also stress and deplete the bicarbonate reserve of the body to neutralize its acidity.

3. The extreme solubility of demineralized water unnaturally increases the absorption of heavy metals from foods and supplements which would otherwise go unabsorbed.

As for acidic water, it's straight-up bad for the kidneys, bones, and the spleen. It will boost the odds of kidney stones, weak bones, and a deteriorating immune system. Try measuring the urine pH to realize the effect. Granted, bicarbonate will buffer it, but the body's capacity to do this is fairly limited.